Werner chromium complexes and methods for their preparation

ABSTRACT

A WERNER COMPLEX COMPOSITON HAS TRIVALENT CHROMIUM ATOMS COORDINATED WITH ACIDO GROUPS AND FLUORINE ATOMS. THE ACIDO GRUPS ARE FROM ALIPHATIC, HYDROXY-CONTAINING, MONOCARBOXYLIC ACIDS HAVING LESS THAN SIX CARBON ATOMS AND NOT MORE THAN THREE HYDROXYL GROUPS. THE COMPLEXES ARE CHARACTERIZED BY A SPECIFIC RANGE FOR THE RATIO OF BOTH THE CHROMIUM ATOMS TO ACIDO GROUPS AS WELL AS CHROMIUM ATOMS TO FLUORINE ATOMS. THE COMPLEX COMPOSITION IS USEFUL IN PREPARING CHROMIUM PLATING BATHS THAT HAVE A BRIGHT PLATING RANGE THAT IS UNIQUE IN THE HISTORY OF DECORATIVE CHROMIUM PLATING.

United States Patent 3,733,347 WERNER CHROMIUM COIVIPLEXES AND METHODS FOR THEIR PREPARATION John Edwin Bride, Mentor, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. N0 Drawing. Filed Feb. 19, 1971, Ser. No. 117,046 Int. Cl. C07f 11/00 U.S. Cl. 260-438.5 C 13 Claims ABSTRACT OF THE DISCLOSURE A Werner complex composition has trivalent chromium atoms coordinated with acido groups and fluorine atoms. The acido groups are from aliphatic, hydroxy-containing, monocarboxylic acids having less than six carbon atoms and not more than three hydroxyl groups. The complexes are characterized by a specific range for the ratio of both the chromium atoms to acido groups as well as chromium atoms to fluorine atoms. The complex composition is useful in preparing chromium plating baths that have a bright plating range that is unique in the history of decorative chromium plating.

BACKGROUND OF THE INVENTION Formerly it has been shown to form complex compounds of the Werner type with trivalent nuclear chromium atoms and acyclic or carbocyclic, carboxylic acido groups, using for the complexing action acids containing ten or more carbon atoms, i.e., typically long chain acids. The chromium atoms could also be further coordinated, with neutral or monovalent groups, but this was of secondary importance. Representative of such groups have been the aquo, chloro, fluoro, bromo, formato, acetate and nitrato groups. The hydrophobic characteristics of surfaces treated with such compositions have been shown, for example, in U.S. Pats. 2,273,040 and 2,356,161.

It has been further shown, for example in U.S. Pat. 2,524,803, that in a special process, other carboxylic acids may be useful, but, because of the nature of the special process disclosed, any halogen present in the complex is chloride. Although the typically long chain acids such as stearic may be used in this special process, and the resulting complex exploited for its hydrophobic quality, shorter chain acids, including monocarboxylic and dicarboxylic acids, and further including unsaturated acids, are also employed. These complexes from the shorter chain acids are of interest for their bonding capabilities, especially to polymers, and thus find utility in sizing glass fiber rovings used as plastic and resin reinforcements.

It has also been heretofore taught that such acids that are not of the long chain type, and which most especially contain amine groups, but can be simple monocarboxylic acids or such acids containing not just one or a few, but very many hydroxyl groups, may be complexed with nuclear trivalent chromium to form compositions of the Werner type. The metal may be further coordinated with neutral groups such as aquo or negative monovalent groups, with chloro, bromo, formato, acetato and nitrato being taught in U.S. Pat. 2,544,668. This patent additionally discloses the desirable bonding between surfaces of diverse materials, particularly polymers, that can be obtained by such Werner complex compounds.

Turning to electrolytic operations, it has been shown that mono-hydroxy, mono-carboxylic acids of short carbon chain are useful in electrolytic polishing baths. Thus US. Pat. 2,645,611 discloses using lactic or glycolic acids as brighteners in an electrolytic polishing bath containing orthophosphoric acid and typically many other ingredients. It has further been taught in U.S. Pat. 3,006,- 823 that hydroxy-containing, mono-carboxylic acids of a few carbon atoms may be complexed with trivalent chromium and the complex may be used to form an aqueous electrolytic plating bath for the plating of bright chromium plate. Such bath can be of interest for the capability of depositing an acceptably bright plate at current densities within the range from 10-200 amperes per square foot (a.s.f.).

SUMMARY OF THE INVENTION The composition of the Werner complex described herein has trivalent chromium atoms coordinated with fluorine atoms and with acido groups of aliphatic, hydroxy-containing, monocarboxylic acid having less than six carbon atoms and not more than three, and preferably only one, hydroxyl group. The complex finds particular utility in the preparation of an aqueous electrolytic plating bath for the deposition of bright chromium plate. The complex provides a decorative plate with a desirable bright finish, shows excellent plating speed in the low current density region, and the bright range is from as high as 950 or more a.s.f., down to about 2 a.s.f., which is unique in the history of decorative chromium plating. The complex of the present invention exhibits desirable Water solubility, plus excellent plating utility over an extended pH range without dissociation or solids precipitation.

Broadly, the invention is directed to a composition of a Werner complex of trivalent chromium atoms coordinated with the acido groups of aliphatic, hydroxy-containing, monocarboxylic acid having less than six carbon atoms and not more than three hydroxyl groups, and providing such complex with a ratio of total chromium atoms to total acido groups within the range of 1:07 to 1:3, such complex being further characterized by containing coordinated fluorine atoms and having a ratio of total chromium atoms to total fluorine atoms within the range of 110.1 to 1:35.

The invention is further directed to the preparation of a composition of a Werner complex containing trivalent chromium atoms coordinated with fluorine atoms and with acido groups of such aliphatic, hydr-oxy-containing, monocarboxylic acid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The complexes of this invention are completely miscible in all proportions in water and exhibit a rich deep greenish color in water solution. Solubility in organic solvents, for example aprotic organic solvents, will vary according to the particular solvent used, but typically will exhibit greater than slight solubility in dimethylformamide and dimethylacetamide. Solutions exhibit extended storage stability and may be neutralized, or changed over to even an alkaline pH, without solids precipitation.

The complexes of this invention can be prepared by any of several methods. One method is the straightforward combination in aqueous medium of chromium metal, preferably in pulverulent form to enhance completion of the reaction, with the aliphatic, hydroxy-containing monocarboxylic acid plus fluorine-providing compound. When particulate metal is used, the reaction can be highly exothermic and thus caution needs to be taken in carrying out such reaction. Typically for enhanced reaction efiiciency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied and is continued with the aqueous medium being heated at reflux to augment completion of the reaction.

The complexes may also be prepared by the blending together of the carboxylic acid and the fluorine-providing compound in an aqueous reaction medium to which there is charged chromic acid, suitably as a solution of chromic acid in water. The chromic acid can be supplied by any of the substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable. In either of these methods, the fluorine-providing substance may be first replaced during reaction by a suitable chlorine-supplying compound such as hydrochloric acid. Such initial reaction therefore provides a complex containing chlorine from which the fluorine-containing complex can be derived by adding to the complex, suitably in the reaction medium and in stoichiometric excess, a fluorine-providing compound and extending the reaction with heating for a time sufficient to displace chlorine atoms in the complex with fluorine atoms.

Other methods can be used to prepare the complex; for example, into about 20 weight parts of water there can be dissolved about 10.5 weight parts of chromium fluoride (CrF -9H O) and, for preparing a complex where the aliphatic, hydroxy-containing, monocarboxylic acid is glycolic acid, typically there can then be added about 5.5 Weight parts of glycolic acid. This can be added, for example, as a solution of 70%, by weight, of glycolic acid and a balance of water. While this mixture is constantly agitated there is gradually added thereto 3.5 weight parts of potassium hydroxide. During the addition of the potassium hydroxide the reaction is exothermic and, as agitation of the mixture is continued, external heating is suitably applied until the mixture is brought to reflux. The mixture can then be maintained at reflux, typically for several hours, and upon cooling an aqueous solution of the complex from the glycolic acid is obtained.

The aliphatic, hydroxy-containing monocarboxylic acids have less than six carbon atoms and not more than three hydroxyl groups and preferably, for economy, have two to four carbon atoms and one hydroxyl group. In addition, these acids are saturated acids, i.e., are free from carbon-to-carbon unsaturation. Thus in addition to glycolic acid the acids of particular interest are the monohydroxy, mono-carboxylic acids such as lactic acid and the mono-hydroxybutyric acids. The source of the fluorine atoms for the complex can be, as mentioned hereinbefore, a fluoride salt of chromium. Other suitable sources of fluorine atoms, especially where the chromium is not present as a fluoride salt, include hydrogen fluoride, sodium fluosilicate, and gaseous fluorine.

Regardless of the acid used and the source of fluorine, the complex should contain a molar ratio of total chrmium atoms to total carboxyl constituent within the range of 1:07 to 1:3, and further contain a molar ratio of total chromium atoms to total fluorine atoms Within the range of 120.1 to 123.5. Especially preferred ratios, which are based upon the utility of the complex in an aqueous plating bath for the deposition of decorative chromium plate, can depend upon the acid constituent of the complex. Thus, for example, in a complex where glycolic acid supplies a substantial amount of the carboxyl constituents, the molar ratio of all of the chromium atoms to all of the acido groups, i.e. to carboxyl constituent, is within the range of 1:19 to 122.6 and the molar ratio for such a complex of all chromium atoms to the total of the fluorine atoms is preferably within the range of about 122.6 to 1:32.

For preparing a chromium plating bath for the deposition of bright chromium plate the complex is generally supplied to a plating medium in an amount providing about 25-150 grams of chromium per liter. The more highly concentrated baths are Well suited for spot plating techniques while the baths containing the lesser concentrations of chromium are useful for immersing therein articles to be plated. Before deposition of chromium such baths are adjusted to a pH within the range from about 1.8 to 4.9 which can be readily handled by the addition of an alkali metal carbonate or hydroxide. The temperature of the bath during plating may range from about 20 C. up to not substantially above about 50 C. for enhanced plating performance.

The bath can also contain a salt of a strong acid preferably, for economy, an alkali metal salt. Such baths may further contain boric acid, or an equivalent to boric acid in aqueous solution, to augment the rate of deposition of the chromium. The alkali metal salts are usually present in an amount of about 50-200 grams per liter of the bath and the boric acid in an amount between about 10-70 grams per liter of the bath. During plating, the object to be plated is made the cathode, for example, immersed in the plating bath, or the cathode in a brush plating operation when the plating medium is contained in a brush, and an inert anode is used, such as a graphite anode. The surfaces that can be plated from such a bath, include metals such as steel, brass, copper, nickel and the like as well as plastic surfaces that are activated or prepared for an electroplating operation. The plating can be carried out in any vessel useful for chromium electroplating such as tanks lined with corrosion resistant materials including glass, ceramic material, polyvinyl chloride and the like.

The invention will be further understood by reference to the following illustrative example which should not be construed as limiting the invention.

Example Into a reaction vessel containing 400 milliliters of water there is dissolved 210 grams of chromium fluoride (CrF -9H 0) and to this is added 153 milliliters of 70% strength glycolic acid which is 70 weight percent glycolic acid and a balance of water. While this mixture is constantly agitated there is gradually added thereto 70 grams of potassium hydroxide. During addition of the potassium hydroxide, the reaction is exothermic and, as agitation of the mixture is continued, external heating is applied until the mixture is brought to boiling. The mixture is then boiled for about three hours and following this the volume of the mixture is adjusted to 750 milliliters.

This resulting chrome complex solution thus prepared contains sufficient complex to provide the solution with 30 grams per liter (g./l.) of chromium. The resulting solution is a clear, deep greenish solution that exhibits excellent extended storage stability without solids precipitation or color change. The solution contains a molar ratio of total chromium to total glycolic acid of 1:2.25 and a molar ratio of all chromium atoms to the total of the fluorine atoms of 112.88. A portion of the chrome complex solution is blended with potassium hydroxide in suflicient amount to change the pH of the portion from acid, past neutral, over to an alkaline pH greater than 8. Upon visual inspection, such alkaline chromic complex solution is seen to be free from color change or visible solids precipitation.

To demonstrate the plating characteristics of such chromium complex solution, suflicient complex solution is prepared as described above to contain 40 g./l. of chromium, and to this is blended 56.6 g./ 1. of H BO and g./l. of potassium chloride, with agitation. The pH of the solution is adjusted to 3.4 and during blending and pH adjustment the volume of the solution is adjusted to 1 liter by the addition of deionized Water.

For this demonstration the resulting solution is placed in a modified Hull cell which is a trapezoidal box of nonconductive material at the opposite ends of which are positioned anode and cathode plates, as has been more particularly described in an article appearing in Plating, vol. 46, No. 3 (1959), page 257. For this cell it is possible to readily determine the effective plating range of a composition under varying conditions. The current density at any point on a cathode is determined according to the formula A==C(27.7-48.7 log L) wherein A is the current density in amps per square foot (a.s.f.) at the selected point, C is the total current in amps applied to the cell, and L is the distance in inches of the selected point from the high current density end of the plate. In the cell graphite anodes are used and the cathode is a 3 x 2 brass panel that is nickel coated prior to use in the cell.

In this cell plating tests are carried out at a bath temperature of 97 F. and at 14 volts using amperes current for a 3 minute cycle. From such tests the bright range of decorative chromium plating is seen to be from about 950 a.s.f. down to 1-2 a.s.f., which is unique in the history of decorative chromium plating. The panel has a relative reflectance value of 50% and the deposit has a desirably rich and deep appearance of a slightly darker cast than that observed with commercially available decorative chromium deposits from hexavalent chromium plating baths.

I claim:

1. A composition of a Werner complex of trivalent chromium atoms coordinated with acido groups of aliphatic, hydroxy-containing, monocarboxylic acid having less than six carbon atoms and not more than three hydroxyl groups, and providing said complex with a ratio of total chromium atoms to total acido groups within the range of 1:0.7 to 1:3, said complex being further characterized by containing coordinated fluorine atoms and having a ratio of total chromium atoms to total fluorine atoms within the range of 1:01 to 1:3.5.

2. A composition as set forth in claim 1 wherein said aliphatic, hydroxy-containing monocarboxylic acid of said Werner complex is glycolic acid.

3. A composition as set forth in claim 1 wherein said aliphatic, hydroxy-containing monocarboxylic acid of said Werner complex is lactic acid.

4. The method for the production of a composition of a Werner complex having trivalent chromium atoms coordinated with fluorine atoms and with acido groups of aliphatic, hydroxy-containing, monocarboxylic acid having less than six carbon atoms and not more than three hydroxyl groups, which method comprises:

( 1) establishing a liquid, aqueous reaction medium containing chromium metal, with sufiicient of said carboxylic acid to provide said medium with a molar ratio of total chromium atoms to total acido groups within the range of 1:07 to 1:3, and with sufficient fluorine-providing compound to supply said reaction medium with a molar ratio of total chromium atoms to total fluorine atoms within the range of 1:0.1 to 123.5; and

(2) permitting reaction of the substituents in said medium.

5. The process of claim 4 wherein said fluorine compound is selected from the group consisting of hydrogen fluoride, sodium fluosilicate, fluorine gas, and mixtures thereof.

6. The process of claim 4 wherein said aliphatic, hydroxy-containing, monocarboxylic acid is selected from the group consisting of glycolic acid, lactic acid, and mixtures thereof.

7. The process of claim 4 wherein water supplies all of the liquid of said reaction medium, such medium is heated to boiling during reaction, and said chromium metal is in pulverulent form.

8. The process for preparing a composition of a Werner complex containing trivalent chromium atoms coordinated with fluorine atoms and with acido groups of aliphatic, hydroxy-containing, monocarboxylic acid having less than six carbon atoms and not more than three hydroxyl groups, which process comprises:

(1) bringing together in water supplying liquid for a reaction medium, chromic acid with suflicient of said carboxylic acid to provide said medium with a molar ratio of total chromium atoms to total acido groups within the range of 1:07 to 1:3, and with fluorineproviding compound sufficient to supply said medium with a molar ratio of total chromium atoms to total fluorine atoms within the range of 1:0.1 to 123.5; and

(2) permitting reaction of the substituents in said medium.

9. The process of claim 8 wherein said fluorine-providing compound is selected from the group consisting of hydrogen fluoride, sodium fiuosilicate, fluorine gas, and mixtures thereof.

10. The process of claim 8 wherein said aliphatic, hydroxy-containing, monocarboxylic acid is selected from the group consisting of glycolic acid, lactic acid, and mixtures thereof.

11. The process for producing a composition of a Werner complex having trivalent chromium atoms coordinated with fluorine atoms and with acido groups of aliphatic, hydroxy-containing, monocarboxylic acids having less than six carbon atoms and not more than three hydroxyl groups, which method comprises:

(1) establishing a liquid, aqueous reaction medium containing chromium-providing material selected from the group consisting of chromium metal, chromic acid, and mixtures thereof, in mixture with said carboxylic acid in amount suflicient to provide said medium with a molar, ratio of total chromium atoms to total acido groups within the range of 1:0.7 to 1:3, and further with chlorine-providing compound sutficient to supply said medium with a molar ratio of total chromium atoms to total chlorine atoms within the range of 1:01 to 113.5;

(2) permitting reaction of the substituents in said medium thereby preparing a composition of a Werner complex having trivalent chromium atoms coordinated with chlorine atoms and acido groups;

(3) furnishing said medium containing said complex with suflicient fluorine-providing compound to supply said medium with a molar ratio of total chromium atoms to total fluorine atoms within the range of 120.1 to 123.5; and

(4) heating the resulting medium at elevated temperature and for a time sufficient to displace chlorine atoms in said complex with fluorine atoms.

12. The process of claim 11 wherein said medium containing said complex is furnished with suflicient fluorineproviding compound to supply therein a stoichiometric excess of fluorine atoms, basis moles of chlorine atoms in said medium.

13. The process of claim 11 wherein said chlorineproviding compound is selected from the group consisting of hydrochloric acid, chlorine gas, and mixtures thereof.

References Cited UNITED STATES PATENTS 2,524,803 10/1950 Iler 260-4385 C 2,825,659 3/1958 Dalton et al 260-4385 C 2,683,156 7/1954 Iler 260-4385 C 3,617,411 11/1971 Couture et a1. 260-4385 C FOREIGN PATENTS 761,834 11/1956 Great Britain 260-4385 C HELEN M. S. SNEED, Primary Examiner US. Cl. X.R. 117-1072 

